This invention relates generally to oxygen or hydrogen transport membranes, and more specifically to oxygen or hydrogen transport membranes fabricated by plasma spray deposition of small particles of ceramic, or metal, or a combination thereof, in order to provide a micro-crack-free coating on a substrate. Also disclosed is a multi-layer composite comprising a dense or porous substrate coated with a coating provided by supersonic plasma spray deposition. Also disclosed is subsonic plasma spray deposition of single phase or dual phase nanocrystalline particles of ceramic or metal, or a combination thereof, to form a crack-free oxygen transport membrane on a substrate.
Oxygen transport membranes (xe2x80x9cOTMsxe2x80x9d) are useful for separating oxygen from gas mixtures containing oxygen, and the OTMs desirably are fabricated using a mixed conductor ceramic membrane, and the resulting membrane desirably has high oxygen selectivity. Illustrative ceramic compositions are disclosed in U.S. Pat. No. 5,342,431 (Anderson et al.); U.S. Pat. No. 5,648,304 (Mazanec et al.); U.S. Pat. No. 5,702,999 (Mazanec et al.); U.S. Pat. No.5,712,220 (Carolan et al.); and U.S. Pat. No. 5,733,435 (Prasad et al.). All of these references are incorporated herein by reference in their entireties. In analogous fashion, other selective ion-transport membranes, such as hydrogen transport membranes (xe2x80x9cHTMsxe2x80x9d) are suitably fabricated to selectively permit hydrogen to pass through the membrane while not allowing other ions to pass.
Thus, the OTMs possess the characteristic of xe2x80x9coxygen selectivityxe2x80x9d, meaning that only oxygen ions are transported across the membrane, with the exclusion of other elements and ions. Likewise, the HTMs possess the characteristic of xe2x80x9chydrogen selectivityxe2x80x9d, meaning that only hydrogen ions are transported across the membrane, with the exclusion of other elements and ions. The OTM""s and HTMs can be fabricated from ceramics, or metals, or a combination thereof. Suitable ceramics for use as the membrane material include single phase mixed conductor perovskites and dual phase metal/metal oxide combinations. Particularly advantageous solid electrolyte ceramic membranes are made from inorganic oxides, typically containing calcium- or yttrium-stabilized zirconium or analogous oxides having a fluorite or perovskite structure. Exemplary ceramic compositions are disclosed in U.S. Pat. No. 5,702,959 to Mazanec et al.; U.S. Pat. No. 5,712,220 to Carolan et al.; and U.S. Pat. No. 5,733,435 to Prasad et al., all of which are incorporated herein by reference in their entirety. The use of such membranes in gas purification applications is described in U.S. Pat. No. 5,733,069 to Prasad et al., which is also incorporated herein by reference in its entirety. Particularly effective OTMs comprise dense films of perovskite-type oxides on porous substrates since these OTMs are particularly good mixed conductors. For example, a computer-simulated model of an OTM having both a dense layer and a contiguous porous layer, wherein the dense layer is said to have no connected through porosity, is disclosed in U.S. Pat. No. 5,240,480 to Thorogood et al., incorporated herein by reference in its entirety. Multilayer OTM composites having, for example, a porous layer and a contiguous denser layer, can have advantageous properties including enhanced oxygen selectivity.
Thin film coatings of oxygen transport membranes, such as the perovskites, are particularly desirable because the ideal oxygen flux is inversely proportional to the thickness of the membrane. On this basis, a thin film is preferred since it permits higher oxygen fluxes and reduced surface area, as compared to thicker films, thus resulting in lower membrane operating temperatures and smaller oxygen pressure differentials across the electrolyte during operation of the membrane.
Heretofore, several techniques have been described for fabricating dense OTM coatings, including chemical vapor deposition, electrostatic spray depositions, electrochemical vapor deposition, sputtering, spray pyrolysis, sol-gel thin film processing, and laser ablation. By way of illustration, a technical journal article by Y. Teraoka et al entitled xe2x80x9cPreparation of Dense Film of Perovskite-Type Oxide on Porous Substratexe2x80x9d, appearing in Nippon Seramkkusu Kyokai Gakiyutsu Ronbunshi (Japanese Version) Volume 97, Nov. 5, 1989 compared sputtering with suspension spray deposition, and concluded that the sputtering process produced a lot of cracks, resulting in a failure to form dense films, whereas improved dense films were formed using the suspension spray deposition technique.
As another illustration, U.S. Pat. No. 5,439,706 to Richards et al discloses a method for manufacturing OTMs using organometallic chemical vapor deposition. Nonetheless, although chemical vapor deposition processes are suitably employed to produce dense, gas-tight ceramic thin films, these processes have the disadvantages of being time consuming, requiring expensive processing equipment, and typically employing toxic precursor chemicals. Further, stoichiometry control for purposes of the formation of the oxide film is difficult to maintain using these processes. The physical deposition processes, such as sputtering and laser ablation, also have distinct disadvantages, since they are typically complex processes, often requiring vacuum systems, and typically employing low deposition rates that don""t lend themselves to use for commercial production of OTMs.
Yet another alternative is thermal spraying. Thermal spraying involves spraying a molten powder of metal or metal oxide onto the surface of a substrate using a plasma or thermal spray gun. In short, it is a heat and momentum-related process involving energy transfer that is attributable to enthalpy and velocity, and the resulting spray coating on a substrate provides strong mechanical bonding of the coating to the substrate without undesirable overheating in view of very short processing times. Thermal spraying processes include plasma spraying and high velocity oxygen fuel (xe2x80x9cHVOFxe2x80x9d) spraying.
Heretofore, plasma spraying has been disclosed for use in fabricating dense lanthanum chromite interconnectors for solid oxide fuel cell applications. Unfortunately, the resulting coating typically is porous and contains significant numbers of microcracks, necessitating a subsequent heat-treating step at temperatures of from 1450 to 1550 degrees Centigrade in order to provide the desired dense, microcrack-free coating. Illustrative of the use of plasma deposition in the preparation of electrically conductive interconnection layers on an electrode structure of an electrochemical cell are the disclosures provided in U.S. Pat. No. 5,391,440 to Kuo et al., incorporated herein by reference in its entirety. There are several disadvantages associated with the invention disclosed in Kuo et al., namely (1) the sub-sonic plasma spraying step provides a coating on the electrode substrate that contains unwanted microcracks, (2) the plasma spray requires the use of a xe2x80x9cfluxxe2x80x9d or xe2x80x9cliquid phase formerxe2x80x9d that risks introducing unwanted elements into the coating, and (3) a post-spraying heat-treating step is required to xe2x80x9chealxe2x80x9d the resulting microcracks.
Heretofore there has been no disclosure of the use of plasma deposition methodology of any kind, much less supersonic plasma deposition, in the preparation of dense, microcrack-free coatings, for use as oxygen transport membranes, without a post plasma spraying heat treating step, to the knowledge of the present inventors. In addition, there has been no disclosure of the use of supersonic plasma deposition to provide microcrack-free thermal barrier or interconnector coatings. Further, there has been no disclosure of the use of subsonic plasma spraying of nanocrystalline particles to produce such microcrack-free coatings.
Fast, cost-effective methods are needed for fabricating dense, thin-film, gas-tight oxygen or hydrogen transport membrane coatings, that are free of microcracks, on dense or porous substrates. The present invention provides one such method for fabricating these OTMs and HTMs using supersonic, sonic or subsonic speed plasma spraying to provide the desired coating.
In one aspect, the present invention relates to a method for fabricating an oxygen or hydrogen transport membrane comprising spraying a single phase ionic, electronic or mixed-conducting or dual phase mixed conducting composition (advantageously comprising particles of ceramic, or metal, or a combination thereof) in plasma form, in the presence of an inert gas as a plasma medium, at a supersonic speed from a plasma torch, and depositing said composition onto a porous or dense substrate in order to provide the oxygen or hydrogen transport membrane in the form of a microcrack-free ceramic coating on the substrate.
In another aspect, the present invention relates to a method of fabricating an oxygen or hydrogen transport membrane comprising spraying particles of a single or dual phase composition comprising particles of ceramic or metal, or a combination thereof, in plasma form, in the presence of an inert gas as a plasma medium, at sonic or subsonic speed from a plasma torch, with the proviso that said particles are agglomerates of nanocrystalline size if the plasma is employed at subsonic speed, and depositing said composition onto a porous or dense substrate in order to provide the oxygen or hydrogen transport membrane comprising a porous or dense ceramic coating on the substrate.
In still another aspect, the present invention relates to a method of forming a dense OTM or HTM coating on a substrate which comprises providing a single or dual phase ceramic-forming feed powder in admixture with an inert gas plasma spray medium, and thermal spraying the feed powder in the plasma spray medium onto a porous or dense substrate in order to form the dense OTM or HTM coating on the substrate.
In yet another aspect, the present invention relates to a microcrack-free oxygen or hydrogen transport membrane produced by a method comprising spraying, at a supersonic speed, particles of a single or dual phase composition comprising particles of ceramic, metal, or a combination thereof, in plasma form, in a flux-free plasma medium of an inert gas, and depositing said composition onto a porous or dense substrate in order to provide the oxygen or hydrogen transport membrane in the form of a microcrack-free coating on the substrate.
In yet another aspect, the present invention relates to a microcrack-free oxygen transport membrane produced by a method comprising spraying single or dual phase powder particles of ceramic, or metal, or a combination thereof, in a plasma medium comprising an inert gas by high velocity oxygen fuel thermal spray at a supersonic speed, and depositing the particles onto a porous or dense substrate in order to provide the oxygen transport membrane in the form of a microcrack-free ceramic coating on the substrate.
In still another aspect, the present invention relates to a multi-layer composite comprising a dense or porous substrate and a microcrack-free coating on the substrate, said coating being prepared by spraying particles of a single or dual phase composition comprising particles of ceramic, or metal, or a combination thereof, in plasma form, in the presence of an inert gas as a plasma medium, at sonic or subsonic speed from a plasma torch, with the proviso that said particles are of nanocrystalline size if the plasma is employed at subsonic speed, and depositing said composition onto a porous or dense substrate in order to provide the multi-layer composite.
In yet another aspect, the present invention relates to a single phase ionic conductor, suitable for use in solid oxide fuel cell or electrically-driven oxygen generator applications, fabricated by spraying single phase powder particles of ceramic, or metal, or a combination thereof, in a plasma medium comprising an inert gas, at supersonic speed, and depositing the particles onto a porous or dense substrate in order to provide the single phase ionic conductor in the form of a microcrack-free ceramic coating on the substrate.
In still another aspect, the present invention relates to a single phase electronic conducting oxide, useful for interconnector applications, fabricated by spraying single phase powder particles of ceramic, in a plasma medium comprising an inert gas, at supersonic speed, and depositing the particles onto a porous or dense substrate in order to provide the single phase electronic conductor in the form of a microcrack-free ceramic or metal coating on the substrate.
These and other aspects will become apparent upon reading the following description of the invention.